The present invention relates to railroad track construction and, more particularly, to a method and apparatus for inserting a fabric web into the rock ballast of a railroad track without disturbing substantially the ties, rails, or ballast of the track.
In conventional railroad track construction, railroad ties are placed in appropriate spaced relation on top of a subgrade. Rails are then spiked to the ties. Ballast material, consisting of crushed rock, is poured onto the ties and rails from a ballast car which moves along rails such that the ballast covers the rails and ties to the top of the rails. The rails and ties are then raised by a specially designed machine. The ballast is swept off the ties such that it fills the spaces between the ties. The ballast is worked in around the ties to provide a firm roadbed and the ballast on each side of the track is contoured.
Several problems have been encountered with railroad roadbeds of conventional construction. As trains pass over a track, the rails, ties, and ballast move up and down slightly, producing in the presence of moisture a pumping action which tends to draw sand and silt out of the substrate soil and up into the rock ballast. The contaminated ballast tends to hold water and, as a result, damage to the ballast from freeze/thaw cycles is increased. Additionally, the moisture retained in the contaminated ballast has an adverse effect upon the life of the wood railroad ties.
An additional problem occurs in regions in which the soil has a high silt content or in which the substrate soil is relatively soft. The pumping action resulting from the passage of trains over the track results in the gradual downward migration of the rock ballast into the substrate soil. As less and less ballast supports the ties, the stability of the roadbed is substantially reduced, thus reducing the speed at which trains may safely travel over the track.
Various approaches have been used to rehabilitate railroad tracks in which ballast has been lost or contaminated. U.S. Pat. No. 2,921,390, issued Jan. 19, 1960, to Stein et al, discloses a ballast plow which is inserted between the roadbed and the ties. The plow is towed along the track in this position by a locomotive and, as it progresses, it raises the ties from the roadbed, supports the ties and the track as it passes underneath and, finally, allows the ties and tracks to resettle onto the roadbed behind the plow. By providing the plow with a scraper blade, it is possible to separate an upper layer of fouled ballast and replace it with fresh ballast before the ties are finally relaid.
U.S. Pat. No. 2,769,172, issued Oct. 30, 1956, to Franco, discloses a sled which is towed beneath the ties and which raises the ties up to their desired level and evenly distributes clean ballast which was previously poured on top of the ties and rails. Additional ballast must thereafter be inserted between adjacent ties. The sled shown in the Franco patent spreads and levels the ballast before permitting the ties to settle onto the ballast behind the sled.
U.S. Pat. No. 2,921,538, and U.S. Pat. No. 2,921,537 to Croonenberghs and Christoff, respectively, both issued Jan. 19, 1960, disclose a plow which is pulled beneath the ties of a railroad track to "skeletonize" the track, that is, to remove the fouled ballast from beneath the track, as the first step in the rehabilitating a section of track. Finally, U.S. Pat. No. 2,575,390, issued Nov. 20, 1951, to McFadden, discloses a ballast working grid which is pulled through the ballast beneath the ties of a railroad track to distribute evenly newly applied ballast.
While such prior art devices may be utilized to reballast a pre-existing railroad track which has lost a portion of its ballast through downward migration of the rock ballast into the soil or in which the ballast has become contaminated through upward movement of silt and sand into the gravel, such rehabilitation techniques are expensive and, additionally, may render sections of the track unusable for extended periods during the rehabilitation and repair process.
In order to reduce the amount of maintenance work required for railroad tracks, a yielding foundation for the ballast has been suggested, as illustrated in U.S. Pat. No. 2,420,333, issued May 20, 1947,to Monroe. In the Monroe foundation, a rubber composition apron is positioned beneath the ballast, with the apron overlying a layer of bituminous cement. The cement, in turn, rests upon a mastic cushion, such as a layer of sand or gravel which is mixed with bitumen. This inhibits the upward migration of water, which would otherwise carry silt into the ballast beneath the railroad ties. The Monroe foundation is practical only for new track construction, since it cannot be added to existing tracks without removing the ties and rails and rebuilding the roadbed.
A more recently developed approach to reducing the problems associated with contamination of rock ballast is to place a sheet of fabric in or beneath the layer of ballast in the railroad roadbed. Typically, a spun polypropylene fabric is utilized, such as any of the following: SUPAC fabric by Philips Perroleum; TYPAR fabric by DuPont; BIDIM fabric by Montsano; and True Tex VT-5000 fabric by True Temper. Such fabrics permit moisture to drain from the ballast, while at the same time preventing upward migration of contaminants.
The use of such fabrics has been relatively limited, however, due to the difficulty of placing the fabric in the ballast of the roadbed of a pre-existing railroad track. In one prior art fabric insertion technique, the rails, ties, and ballast were removed prior to laying down a web of the fabric on the roadbed. Alternatively, a ballast plow has been used for removing ballast from a section of the track and a small roll of the fabric was placed beneath the ties and unrolled, prior to reballasting the track section. Such known techniques for inserting the fabric into the roadbed of a pre-existing track are relatively expensive and time consuming and, as a consequence, have not been widely accepted.
Accordingly, there is a need for a method and apparatus capable of inserting a web of fabric into the ballast of a railroad roadbed without substantially distrubing the tracks, ties, or ballast.